There are many examples of functional elements or components which can provide, produce, or detect electromagnetic signals or other characteristics. These functional components may be used to make electronic devices. One example of such devices is that of a radio frequency (RF) identification tag (RFID tag) which contains a chip (an integrated circuit, or IC) or several chips that are formed with a plurality of electronic elements. Information is recorded into these chips, which may then be relayed to a base station. Typically, this is accomplished as the RFID tag, in response to a coded RF signal received from the base station, functions to cause the tag to reflect the incident RF carrier back to the base station thereby transferring the information.
Functional components such as semiconductor chips having an RF circuit, logic and memory have been incorporated into an RFID tag. Such a tag may also have an antenna, as well as a collection of other necessary components such as capacitors or a battery, all mounted on a substrate and sealed with another layer of material. The antenna material can be a thin film metal which can be deposited on a substrate. Alternatively, the antenna material can also be affixed to the substrates using adhesive.
One type of process for manufacturing an electronic assembly, such as an RFID tag or label, involves a web process. In a web process, a web material of a carrier substrate is advanced through a web process apparatus using rollers. Functional components (blocks) are deposited on the carrier substrate web, for example, by using a fluidic self assembly (FSA) process, as described in U.S. Pat. No. 5,545,291, which is herein incorporated by reference. The FSA process deposits a plurality of functional blocks onto the web material wherein the blocks fall into recessed regions found in the web material. At a further point in the web process, a flexible carrier strap comprising the functional components (e.g. RFID chips) of the carrier substrate is coupled to a web of receiving substrate which also comprises functional components, such as antennas. Further details of such a manufacturing process are described in U.S. Pat. No. 6,606,247, which is herein incorporated by reference. The assembly of these components often requires complex and multiple processes, which can cause the end product to be expensive. Accordingly, it is advantageous for manufacturing processes to provide satisfactory production yields.
One such factor which can adversely affect production yields during assembly of RFID tags is the risk of damaging electrostatic discharge events. Generally, static electricity is an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance. Electrostatic discharge (ESD) is the transfer of charge between objects at different electrical potentials. ESD can change the electrical characteristics of a functional component or semiconductor device, thereby degrading or destroying it. When an ESD sensitive electronic device, such as an integrated circuit (IC) in an RFID tag, is exposed to an ESD event, it may negatively impact the RF performance of the device, or the device may even no longer function (i.e. catastrophic failure). For example, a damaging ESD event may cause a metal melt, junction breakdown, oxide failure, or other permanent damage to the device's circuitry causing the device to fail.
Electrostatic charge is most commonly created by the contact and separation of two materials. Creating electrostatic charge by contact and separation of materials is known as “triboelectric charging,” which involves the transfer of electrons between materials. An example of triboelectric charging is illustrated by FIGS. 1A and 1B. The atoms of a material with no static charge have an equal number of positive (+) protons in their nucleus and negative (−) electrons orbiting the nucleus. FIG. 1A illustrates an example of two uncharged materials coming into contact with each other. In FIG. 1A, Material A consists of atoms with equal numbers of protons and electrons. Material B also consists of atoms with equal (though perhaps different) numbers of protons and electrons. Both materials are electrically neutral. When the two materials are placed in contact and then separated, as illustrated in FIG. 1B, negatively charged electrons may be transferred from the surface of one material to the surface of the other material. Which material loses electrons and which gains electrons will depend on the nature of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons is negatively charged. A charge (q) on an object creates an electrostatic potential between itself and another object that is the quotient of the charge difference between the objects (Δq) and the capacitance (C) between the objects, i.e. V=Δq/C. This is expressed in voltage. Furthermore, the system of the two objects has a potential energy equal to the product of the charge and the voltage. The swift release of this potential energy is the source of the ESD damage. When the potential difference exceeds the breakdown voltage of the air between the objects, a charge moves through the air to neutralize the charge difference. This is the ESD event. As described above, this potential energy can be released over very short time scales. Thus, the instantaneous power can be very high, which may result in fusing metal, perforating junctions, or other types of damage.
The process of material contact, electron transfer and separation, is often a more complex mechanism than described above. The amount of charge created by triboelectric generation is affected by the area of contact, the speed of separation, and relative humidity, material types among other factors. Once the charge is created on a material, it becomes an “electrostatic” charge if it remains on the material. This charge may be transferred from the material, creating an electrostatic discharge (ESD) event. Additional factors such as the resistance of the actual discharge circuit, the contact resistance at the interface between contacting surfaces and the capacitance between the objects also affect the magnitude of the current that can cause damage. An electrostatic charge also may be created on a material in other ways such as by static induction, ion bombardment, or contact with another charged object. However, triboelectric charging is often the most common.
FIG. 2 illustrates an example of a roll to roll web processing apparatus 200, such as that used to manufacture RFID tags and labels. Generally, as described herein, an RFID tag includes an RFID IC attached to an antenna structure. A web material having a plurality of antennas attached thereto may be referred to as antenna stock. An RFID inlet (also known as an RFID Inlay) is an antenna with an attached strap. An inlet web is a web of antennas with attached straps. Further, an RFID label is, for example, an inlet covered on one side by a paper covering and an adhesive backing on the other side. An inlet with a covering on the antenna side may be referred to as an RFID tag. The RFID label may be singulated, or may still be on the web material (e.g. a roll of many RFID labels). RFID tags and labels are manufactured in a conversion process in which a web of material goes through many process steps. Label converters are processing apparatuses that convert the inlet web into the final RFID labels. FIG. 2 may be considered to represent a case in which an antenna stock is being processed in a roll to roll manufacturing operation which includes the attachment of RFID ICs (e.g. RFID ICs carried on straps) onto the antennas on the antenna stock. For example, the web of material may have conductive antennas adhered to it at a fixed pitch. This web will naturally build up large static charges on its surfaces as it comes into contact with other materials and is disengaged, such as when it passes over a roller.
During the manufacturing process, as a carrier web 201 passes over various rollers 204, 206, there are multiple opportunities for electrostatic discharge (ESD) events to occur. The carrier web 201 has small conductive functional components 202 (e.g. an RFID IC coupled to an antenna) deposited on a front side 203 of the web 201. In the simplest case, the web 201 passes over a roller 204 so that the non-conductor side 205 contacts the roller 204. The roller 204 is typically metallic. The web 201 may be unrolling from a spindle or roll at this point, or it maybe passing over the roller 204 from another process. It should be noted that triboelectrification can occur when two layers of a material are separated from each other. The web 201 is often made of a plastic material, with functional components 202 deposited thereon on one side 203. Since the web 201 and roller 204 are in contact with each other and then separate, there is an opportunity for triboelectrification, as described above. In this example, positive charges 207 occur on the roller 204 and negative charges 208 occur on the backside 205 of the web 201. The functional components 202 are conductive islands, and will experience a negative electrostatic potential.
As these components 202 approach a metallic roller 206, there may be a sudden transfer of charge, or an electrostatic discharge (ESD) event 210. This typically occurs when the air gap between a component 202 and the roller 206 is so small that the resulting field is greater than the breakdown field of air. Since this is often a metal-to-metal discharge, the ESD is a fast rising (e.g. about 100 picoseconds), narrow (e.g. less than about 500 picoseconds) current pulse. For approximately a 5000 volt static potential, the peak current may exceed about 50 amperes. This high instantaneous power may damage or render the functional components 202 inoperable, as described above, leading to a decreased production yield. Accordingly, it is desirable to control and reduce the likelihood of such damaging ESD events during the production of RFID inlets, tags and labels.